They're having one of their special days on Sunday, including the little steam engine for kiddie rides 50 cents and a steam train going from Hyvinkää to Riihimäki (I think) adults 10€/kids 3€ and I assume it will bring you back again. There's also a meccano exhibition on at the moment.
Oh I'll be there, if you wish to speak to/avoid me, I'm the one hobbling around (pulled a muscle) with a 5 year old girl that never stops talking.